The present invention relates to multilayer magnetic superlattice-based, perpendicular magnetic recording media which have improved thermal stability and very high perpendicular magnetic coercivities which can exceed about 6,500 Oe. The invention finds particular utility in the manufacture of very high areal recording density magnetic data/information storage and retrieval media such as hard disks and hybrid recording devices and systems including such media in combination with magnetoresistive read heads and inductive write heads.
Magnetic recording media and devices incorporating same are widely employed in various applications, particularly in the computer industry for data/information storage and retrieval purposes, typically in disk form. Conventional magnetic thin-film media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording medium layer, are generally classified as xe2x80x9clongitudinalxe2x80x9d or xe2x80x9cperpendicularxe2x80x9d, depending upon the orientation of the magnetic domains of the grains of magnetic material.
Efforts are continually being made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, of the magnetic media. However, severe difficulties, such as thermal instability, are encountered when the bit density of longitudinal media is increased above about 20-50 Gb/in2 in order to form ultra-high recording density media, when the necessary reduction in grain size exceeds the superparamagnetic limit. Such thermal instability can, inter alia, cause undesirable decay of the output signal of hard disk drives, and in extreme instances, result in total data loss and collapse of the magnetic bits. In this regard, the perpendicular recording media have been found superior to the more common longitudinal media in achieving very high bit densities.
As indicated above, much effort has been directed toward enhancing the density of data storage by both types of magnetic media, as well as toward increasing the stability of the stored data and the ease with which the stored data can be read. For example, it is desirable to develop magnetic media having large magnetic coercivities, Hc, since the magnetic moments of such materials require large magnetic fields for reorientation, i.e., switching between digital 1 and 0. Thus, when the magnetic medium has a large coercivity Hc, exposure of the magnetic medium to stray magnetic fields, such as are generated during writing operations, is less likely to corrupt data stored at adjacent locations.
The density with which data can be stored within a magnetic thin-film medium for perpendicular recording is related to the perpendicular anisotropy (xe2x80x9cKuxe2x80x9d or xe2x80x9cK11xe2x80x9d) of the material, which reflects the tendency for the magnetic moments to align in the out-of-plane direction. Thin-film magnetic media having high perpendicular anisotropy have their magnetic moments aligned preferentially perpendicular to the plane of the thin film. This reduces the transition length between the magnetic moments with opposite direction, thereby allowing a larger number of magnetic bits (domains) to be packed into a unit area of the film and increasing the areal density with which data can be stored.
A large perpendicular anisotropy is also reflected in a larger value of the magnetic coercivity Hc, since the preferential out-of-plane alignment of the magnetic moments raises the energy barrier for the nucleation of a reverse magnetization domain, and similarly, makes it harder to reverse the orientation of the magnetic domains by 180xc2x0 rotation. Further, the magnetic remanence of a medium, which measures the tendency of the magnetic moments of the medium to remain aligned once the magnetic field is shut off following saturation, also increases with increasing K1.
While perpendicular media have been fabricated utilizing a single perpendicularly oriented magnetic recording layer, a promising new class of materials for use as the active recording layer of perpendicular magnetic media includes multilayer magnetic xe2x80x9csuperlatticexe2x80x9d structures comprised of a stacked plurality of very thin magnetic/non-magnetic layer pairs, for example, cobalt/platinum (Co/Pt)n and cobalt/palladium (Co/Pd)n multilayer stacks. As schematically illustrated in the cross-sectional view of FIG. 1, such multilayer stacks or superlattice structures 10 comprise n pairs of alternating discrete layers of Co or Co-based materials (designated by the letter A in the drawing) and Pt or Pd (designated by the letter B in the drawing), where n is an integer between about 5 and about 50. Superlattice 10 is typically formed by suitable thin film deposition techniques, e.g., sputtering, and can exhibit perpendicular magnetic isotropy arising from metastable chemical modulation in the direction normal to the underlying substrate S on which superlattice 10 is formed. Compared to media with a single layer of conventional cobalt-chromium (Co-Cr) magnetic alloys utilized in magnetic data storage/retrieval disk applications, such (Co/Pt)n and (Co/Pd)n, multilayer magnetic superlattice structures offer a number of performance advantages. For example, a sputtered (Co/Pt)n multilayer stack or superlattice 10 suitable for use as a magnetic recording layer of a perpendicular medium can comprise n Co/Pt or Co/Pd layer pairs, where n=about 5 to about 50, e.g., 20, and wherein each Co/Pt layer pair consists of an about 3 xc3x85 thick Co layer adjacent to an about 10 xc3x85 thick Pt or Pd layer, for a total of 40 separate (or discrete) layers, and are characterized by having a large perpendicular anisotropy, high coercivity Hc, and a high squareness ratio of a magnetic hysteresis (M-H) loop measured in the perpendicular direction. By way of illustration, (Co/Pt)n and (Co/Pd)n multilayer magnetic superlattices, wherein n=about 10 to about 30 and the layer pairs have thicknesses as indicated above and fabricated, e.g., by means of techniques disclosed in U.S. Pat. No. 5,750,270, the entire disclosure of which is incorporated herein by reference, have stable magnetic domains with a narrow domain wall width, the stability of the Co/Pt and Co/Pd domains being enhanced by a strong domain wall pinning effect. Thus, (Co/Pt)n and (Co/Pd)n multilayer superlattice structures for use in the fabrication of magneto-optical (xe2x80x9cMOxe2x80x9d) recording media, perpendicular recording media, and/or magnetoresistance (xe2x80x9cMRxe2x80x9d) recording media exhibit perpendicular anisotropies exceeding about 2xc3x97106 erg/cm3; coercivities as high as about 5,000 Oe; squareness ratios (S) of a M-H loop, measured in the perpendicular direction, of from about 0.85 to about 1.0, and carrier-to-noise ratios (xe2x80x9cCNRxe2x80x9d) of from about 30 dB to about 60 dB.
Multilayer superlattice-based structures provide a number of additional advantages vis-à-vis conventional thin-film magnetic media. For example, by virtue of their small magnetic domain diameters, they can advantageously support high areal recording densities (e.g., xcx9c100-600 Gb/in2 at domain diameters less than 20 nm); they can be configured either in MR or MO type drives or employed in the form of xe2x80x9chybridxe2x80x9d recording devices including magnetoresistive or giant magnetoresistive (xe2x80x9cGMRxe2x80x9d) read heads (see below) and inductive write heads; and the read-back signal can be differentiated, whereby the sharp rise-time of the differentiated signal further facilitates high areal density recording.
A key advance in magnetic recording technology which has brought about very significant increases in the data storage densities of magnetic disks has been the development of extremely sensitive magnetic read/write devices which utilize separate magnetoresistive read heads and inductive write heads. The magnetoresistive effect, wherein a change in electrical resistance is exhibited in the presence of a magnetic field, has long been known; however, utilization of the effect in practical MR devices was limited by the very small magnetoresistive response of the available materials. The development in recent years of materials and techniques (e.g., sputtering) for producing materials which exhibit much larger magnetoresistive responses, such as Fe-Cr multilayers, has resulted in the formation of practical read heads based upon what is termed the giant magnetoresistive effect, or xe2x80x9cGMRxe2x80x9d. Further developments in GMR-based technology have resulted in the formation of GMR-based head structures, known as GMR xe2x80x9cspin valvexe2x80x9d heads, which advantageously do not require a strong external magnet or magnetic field to produce large resistance changes, and can detect weak signals from tiny magnetic bits.
The use of such GMR-based spin valve heads can significantly increase the areal density of magnetic recording media and systems by increasing the track density, as expressed by the number of tracks per inch (xe2x80x9cTPIxe2x80x9d) and the linear density, as expressed by the number of bits per inch (xe2x80x9cBPIxe2x80x9d), where areal density=TPIxc3x97BPI. Currently, GMR-based spin valve heads are utilized for obtaining areal recording densities of more than about 10 Gb/in2; however, even greater recording densities are desired. A difficulty encountered with further increase in the BPI of conventional magnetic recording media is that the smaller grain sizes necessary for increase in the BPI results in thermal instability of the media due to exceeding the superparamagnetic limit.
As indicated supra, sputtered multilayer magnetic superlattice structures can provide several advantages vis-à-vis conventional thin-film magnetic recording media, when utilized in fabricating very high areal density media. Specifically, multilayer magnetic superlattice magnetic recording media exhibit higher interfacial (i.e., perpendicular) anisotropy (Ku or K1) than conventional thin film media, e.g., greater values of KuV/kT (where Ku=anisotropy constant; V=volume in cm3; k=Boltzmann""s constant; and T=absolute temperature, xc2x0 K); increased thermal stability; and very high values of perpendicular coercivity Hc, i.e.,  greater than 103 Oe. In this regard, the very high values of Hc attainable with multilayer magnetic superlattice structures translates into a significant increase in BPI, and thus a substantial increase in areal recording density. In order to satisfactorily perform write operations on such high coercivity media, it is necessary that the material of the write head have a high saturation magnetic moment (Bsat), which is preferably three (3) times that of the coercivity Hc, i.e., from about 12,000 to about 30,000 Gauss for Hc in the range of from about 4,000 to about 10,000 Oe, with a strong inductive flux. The high Bsat material is also utilized for an underlying soft magnetic (xe2x80x9ckeeperxe2x80x9d) layer of the medium. Materials such as NiFe, CoNiFe, CoZrNb, FeTaC, FeCoB, and FeAlN are usable for this purpose, with FeAlN being preferred in view of its very high value of Bsat.
Multilayer magnetic superlattice structures utilized in magnetic recording media can have either an ordered structure forming a true superlattice, or a disordered structure, variously termed a xe2x80x9cnon-superlattice multilayerxe2x80x9d or a xe2x80x9cpseudo-superlattice structurexe2x80x9d. In (Co/Pt)n and (Co/Pd)n multilayer superlattice structures having utility in high areal recording density media, the interfaces between the Co and Pt (or Pd) layers incur surface interactions, such as for example, spin-orbit coupling. Because Co and Pt (or Pd) have different electronic shell structures or configurations, spin-orbit coupling occurs between the spin and orbital motions of the electrons. When an external magnetic field is applied for reorienting the spin of an orbiting electron, the orbit is also reoriented. However, because the orbit is strongly coupled to the metal lattice, the spin axis of the electron resists rotation. The energy required for reorienting (i.e., rotating) the spin system of a magnetic domain away from the easy direction is termed the anisotropy energy. The stronger the spin-orbit coupling or interaction, the higher the anisotropy energy and the coercivity Hc.
It is believed that an increase in the interfacial anisotropy of (Co/Pt)n and (Co/Pd)n multilayer magnetic superlattice structures, as by an increase in the disorder or xe2x80x9cbroken symmetryxe2x80x9d at each of the Co/Pt or Co/Pd interfaces, can result in the obtainment of very high perpendicular magnetic coercivities Hc necessary for fabricating ultra-high areal density, thermally stable magnetic recording media, i.e., perpendicular coercivities on the order of about 104 Oe. It is further believed that the amount or degree of disorder or xe2x80x9cbroken symmetryxe2x80x9d at each of the Co/Pt or Co/Pd interfaces of sputtered (Co/Pt)n and (Co/Pd)n superlattices can be increased by substituting the conventionally employed Ar sputtering gas with a higher atomic weight sputtering gas, e.g., Kr or Xe, thereby providing bombardment of the deposited films with ionized particles having greater momentum, resulting in a greater amount of lattice disruption, disorder, and/or xe2x80x9cbroken symmetryxe2x80x9d at the layer interfaces, as disclosed in U.S. Pat. No. 5,106,703 to Carcia, the entire disclosure of which is incorporated herein by reference.
Referring again to FIG. 1, according to conventional practices for forming multilayer magnetic superlattice structures 10 in the fabrication of magnetic recording (MR) media, a layer of a xe2x80x9csoftxe2x80x9d magnetic material, i.e., where Hc less than 1 Oe (alternatively referred to as a xe2x80x9ckeeperxe2x80x9d or underlayer), and at least one overlying thin layer, termed a xe2x80x9cseed layerxe2x80x9d, typically comprising at least one material selected from Pt, Pd, Ag, Au, Rh, Ir, Cu, and Mn, are interposed between the substrate S and the xe2x80x9chardxe2x80x9d multilayer magnetic superlattice structure 10 (i.e., where Hc greater than 4,000 Oe) for, inter alia, performing writing operations and enhancing/controlling the crystallographic texture and grain size/structure of the latter.
A frequently encountered problem in the preparation of multilayer magnetic superlattice structures for use in large scale, automated fabrication of very high recording density magnetic media for use in, e.g., hybrid recording systems utilizing GMR heads, is difficulty in reliably and controllably achieving a desired interfacial anisotropy of the magnetic superlattice, hence a desired high thermal stability and perpendicular magnetic coercivity Hc.
Accordingly, there exists a need for improved perpendicular magnetic recording media, which media are based on multilayer magnetic superlattice structures (as described supra), which exhibit very high and readily controllable perpendicular magnetic coercivities Hc, i.e., from at least about 4,000 Oe to greater than about 6,500 Oe, by methodology which can be easily and readily implemented in a cost-effective manner for fabrication of very high areal recording density media. Further, there exists a need for improved perpendicular media having very high areal recording densities and improved thermal stability for use in hybrid recording systems, which media can be fabricated in an economical fashion utilizing conventional automated manufacturing equipment.
The present invention is based upon the discovery that effective regulation and control of the perpendicular magnetic coercivity Hc of multilayer magnetic superlattice structures for use as recording layers in high areal density perpendicular magnetic recording media, such as described supra, at Hc values of from at least about 4,000 Oe to greater than about 6,500 Oe, can be accomplished by forming (as by sputtering) on a suitable substrate a multilayer structure, termed a xe2x80x9cHASxe2x80x9d structure, which HAS structure is comprised, in sequence, of (1) a xe2x80x9chardxe2x80x9d (xe2x80x9cHxe2x80x9d) ferromagnetic recording layer (i.e., Hc greater than 4,000 Oe, as previously defined), which may be of single or multilayer superlattice construction; (2) an anti-ferromagnetic (xe2x80x9cAxe2x80x9d) coupling layer structure; and (3) a xe2x80x9csoftxe2x80x9d (xe2x80x9cSxe2x80x9d) magnetic xe2x80x9ckeeperxe2x80x9d or underlayer (i.e., Hc less than 1 Oe, as previously defined) formed on the substrate, which soft underlayer material has a very high value of Bsat. Moreover, the improved perpendicular magnetic media of the present invention exhibit very high areal recording densities along with improved thermal stability, and thus are suitable for use in hybrid recording systems, while maintaining full compatibility with all technological aspects of conventional magnetic recording media. Further, the methodology provided by the present invention enjoys diverse utility in the manufacture of all manner of films, devices, and products requiring multilayer magnetic thin film coatings and structures exhibiting very high values of perpendicular anisotropy and magnetic coercivity with good thermal stability.
An advantage of the present invention is a multilayer magnetic superlattice-based perpendicular magnetic recording medium having improved thermal stability and perpendicular magnetic coercivity.
Another advantage of the present invention is an improved, high areal recording density perpendicular magnetic recording density.
Yet another advantage of the present invention is a high areal recording density, multilayer magnetic superlattice-based, perpendicular magnetic recording medium including anti-ferromagnetically coupled soft magnetic xe2x80x9ckeeperxe2x80x9d and hard magnetic recording layers.
Still another advantage of the present invention is a perpendicular magnetic recording medium having improved thermal stability and high perpendicular coercivity which can exceed about 6,500 Oe.
Additional advantages and other features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to one aspect of the present invention, the foregoing and other advantages are obtained in part by a high areal recording density, perpendicular magnetic recording medium, comprising:
(a) a non-magnetic substrate having a surface with a layer stack formed thereon, the layer stack comprising, in overlying sequence from the substrate surface:
(b) a soft ferromagnetic xe2x80x9ckeeperxe2x80x9d or underlayer;
(c) an anti-ferromagnetic coupling layer structure; and
(d) a hard ferromagnetic perpendicular recording layer including a multilayer magnetic superlattice structure;
wherein layers (b), (c), and (d) in stacked combination provide the medium with improved thermal stability and a high perpendicular coercivity which can exceed about 6,500 Oe.
In accordance with embodiments of the present invention, the medium further comprises:
(e) an adhesion layer between the surface of the non-magnetic substrate and the soft ferromagnetic underlayer;
(f) a seed layer between the soft ferromagnetic underlayer and the anti-ferromagnetic coupling layer structure;
(g) a protective overcoat layer over the hard ferromagnetic perpendicular recording layer; and
(h) a lubricant topcoat over the protective overcoat layer.
According to embodiments of the present invention, the non-magnetic substrate (a) comprises a non-magnetic material selected from the group consisting of Al, Al-based alloys, NiP-plated Al, other non-magnetic metals, other non-magnetic metal alloys, glass, ceramics, glass-ceramics, polymers, and laminates and composites thereof; the soft magnetic underlayer (b) is from about 1,500 to about 5,000 xc3x85 thick, has a saturation moment Bsat as high as about 30,000 Gauss, and comprises a material selected from the group consisting of NiFe, CoNiFe, CoZrNb, FeTaC, FeCoB, and FeAlN; and the anti-ferromagnetic coupling layer structure (c) comprises a layer stack composed of at least one ferromagnetic layer of Co and at least one anti-ferromagnetic layer of Ru, i.e., the anti-ferromagnetic coupling layer structure (c) comprises a layer stack selected from the group consisting of Co/Ru, Ru/Co/Ru, and Ru/Co/Ru/Co layer stacks, wherein each Co layer is about 3 xc3x85 thick and each Ru layer is from about 8 to about 10 xc3x85 thick.
According to specific embodiments of the present invention, the hard ferromagnetic perpendicular recording layer (d) comprises a stacked multilayer superlattice structure comprising n Co-based magnetic/Pd- or Pt-based non-magnetic layer pairs, where n is an integer between from about 5 to about 50, each Co-based magnetic layer is from about 3 to about 5 xc3x85 thick, each Pd- or Pt-based non-magnetic layer is from about 7 to about 10 xc3x85 thick, and the superlattice structure is selected from the group consisting of (Co/Pd)n, (Co/Pt)n; (CoCrX/Pd)n, where X=Ta, B, Mo or Pt; and (CoCrX/Pt)n, where X=Ta, B, Mo or Pt.
In accordance with further embodiments of the present invention, the adhesion layer (e) has a thickness of from about 25 to about 50 xc3x85 and comprises a material selected from the group consisting of Cr and Ti; the seed layer (f) comprises a Ta/ITO layer pair wherein the thickness of each of the Ta and ITO layers is about 5 nm; or the seed layer (f) comprises an about 3 to about 50 xc3x85 thick layer of a material selected from the group consisting of Pd, Pt, Si, and SiN; the protective overcoat layer (g) is from about 25 to about 60 xc3x85 thick and comprises a wear-resistant material selected from the group consisting of diamond-like carbon (xe2x80x9cDLCxe2x80x9d); sputtered a-C:H, a-C:HN, and a-C:N; ion beam deposited carbon (xe2x80x9cIBD-Cxe2x80x9d); cathodic arc-deposited carbon (xe2x80x9cCAD-Cxe2x80x9d); SiN, AlN, SiC, SiN/C, AlN/C, and SiC/C; and the lubricant topcoat layer (h) is from about 3 to about 30 xc3x85 thick and comprises a high molecular weight fluoropolyether or perfluoropolyether.
According to another aspect of the present invention, a high areal recording density, perpendicular magnetic recording medium, comprises:
(a) a non-magnetic substrate having a surface with a layer stack formed thereon, the layer stack comprising, in overlying sequence from the substrate surface:
(b) a soft ferromagnetic xe2x80x9ckeeperxe2x80x9d or underlayer, the soft magnetic underlayer (b) being from about 1,500 to about 5,000 xc3x85 thick, having a saturation moment Bsat as high as about 30,000 Gauss, and comprising a material selected from the group consisting of NiFe, CoNiFe, CoZrNb, FeTaC, FeCoB, and FeAlNx,
(c) an anti-ferromagnetic coupling layer structure, the anti-ferromagnetic coupling layer structure (c) comprising a layer stack composed of at least one ferromagnetic layer of Co and at least one anti-ferromagnetic layer of Ru, the layer stack being selected from the group consisting of Co/Ru, Ru/Co/Ru, and Ru/Co/Ru/Co layer stacks, wherein each Co layer is about 3 xc3x85 thick and each Ru layer is from about 8 to about 10 xc3x85 thick; and
(d) a hard ferromagnetic perpendicular recording layer comprised of a multilayer magnetic superlattice structure;
wherein layers (b), (c), and (d) in stacked combination provide the medium with improved thermal stability and a high perpendicular coercivity which can exceed about 6,500 Oe.
In accordance with specific embodiments of the present invention, the hard ferromagnetic perpendicular recording layer (d) comprises a stacked multilayer in the form of a superlattice structure comprising n Co-based magnetic/Pd- or Pt-based non-magnetic layer pairs, where n is an integer between from about 5 to about 50, each Co-based magnetic layer is from about 3 to about 5 xc3x85 thick, each Pd- or Pt-based non-magnetic layer is from about 7 to about 10 xc3x85 thick, and the superlattice structure is selected from the group consisting of (Co/Pd)n; (Co/Pt)n; (CoCrX/Pd)n, where X=Ta, B, Mo or Pt; and (CoCrX/Pt)n, where X=Ta, B, Mo or Pt.
According to further embodiments of the present invention, the medium further comprises:
(e) an adhesion layer between the surface of the non-magnetic substrate and the soft ferromagnetic underlayer;
(f) a seed layer between the soft ferromagnetic underlayer and the anti-ferromagnetic coupling layer structure;
(g) a protective overcoat layer over the hard ferromagnetic perpendicular recording layer; and
(h) a lubricant topcoat over the protective overcoat layer.
In accordance with specific embodiments of the present invention, the adhesion layer (e) has a thickness of from about 25 to about 50 xc3x85 and comprises a material selected from the group consisting of Cr and Ti; the seed layer (f) comprises a Ta/ITO layer pair wherein the thickness of each of the Ta and ITO layers is about 5 nm; or the seed layer (f) comprises an about 3 to about 50 xc3x85 thick layer of a material selected from the group consisting of Pd, Pt, Si, and SiN; the protective overcoat layer (g) is from about 25 to about 60 xc3x85 thick and comprises a wear-resistant material selected from the group consisting of diamond-like carbon (xe2x80x9cDLCxe2x80x9d); sputtered a-C:H, a-C:HN, and a-C:N; ion beam deposited carbon (xe2x80x9cIBD-Cxe2x80x9d); cathodic arc-deposited carbon (xe2x80x9cCAD-Cxe2x80x9d); SiN, AlN, SiC, SiN/C, AlN/C, and SiC/C; and the lubricant topcoat layer (h) is from about 3 to about 30 xc3x85 thick and comprises a high molecular weight fluoropolyether or perfluoropolyether.
Still another aspect of the present invention is a perpendicular magnetic recording medium, comprising:
a non-magnetic substrate having a surface with a layer stack formed thereon; and
means for providing anti-ferromagnetic coupling between a soft ferromagnetic xe2x80x9ckeeperxe2x80x9d or underlayer and a hard ferromagnetic, multilayer superlattice-based, perpendicular recording layer of the layer stack.
According to embodiments of the present invention, the medium further comprises:
an adhesion layer between the surface of the non-magnetic substrate and the soft ferromagnetic xe2x80x9ckeeperxe2x80x9d or underlayer; and
a seed layer between the soft ferromagnetic xe2x80x9ckeeperxe2x80x9d or underlayer and the means for providing anti-ferromagnetic coupling;
a protective overcoat layer over the hard ferromagnetic perpendicular recording layer; and
a lubricant topcoat layer over the protective overcoat layer.
In accordance with embodiments of the present invention, the hard ferromagnetic perpendicular recording layer comprises a multilayer magnetic superlattice structure including n Co-based magnetic/Pd- or Pt-based non-magnetic layer pairs, where n is an integer between from about 5 to about 50, each Co-based magnetic layer is from about 3 to about 5 xc3x85 thick, each Pd- or Pt-based non-magnetic layer is from about 7 to about 10 xc3x85 thick, and the superlattice structure is selected from the group consisting of (Co/Pd)n; (Co/Pt)n; (CoCrX/Pd)n, where X=Ta, B, Mo or Pt; and (CoCrX/Pt)n, where X=Ta, B, Mo or Pt.
Additional advantages and aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein embodiments of the present invention are shown and described, simply by way of illustration of the best mode contemplated for practicing the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects, all without departing from the spirit of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as limitative.